Recently there have been demands for improving wiring densities and performances of printed circuit boards with miniaturization and performance enhancement of electronics devices. Polyimide resins are excellent in properties such as heat resistance, electrical insulation property, mechanical strength, and flexibility, and thereby have been practically used in base films for flexible printed boards and TAP tapes. Generally the polyimide resins have been widely used such that films thereof are covered with copper to form copper-clad polyimide substrates.
Known methods for producing the copper-clad polyimide substrates include laminating methods, casting methods, and sputtering/plating methods. In the laminating methods, a polyimide film is attached to a copper foil by using an adhesive, and in the casting methods, a polyimide precursor of a polyamic acid is applied to a roughened copper foil surface and heated. Though these methods are advantageously excellent in productivity and adhesion between a copper foil and polyimide, the interface between the copper foil and polyimide is largely rough and it is difficult to reduce the copper foil thickness, so that the methods are disadvantageously unsuitable for fine patterning.
In contrast, the sputtering/plating methods are such that a conductive layer is formed on a polyimide film surface by sputtering and a copper layer is thickened by electroplating. Thus, the interface between the copper layer and the polyimide is flat, and the thickness of the copper layer can be controlled in the electroplating, whereby the methods are suitable for fine patterning.
However, an expensive sputtering apparatus is needed in the sputtering/plating methods, resulting in increased costs of the copper-clad polyimide substrate and low productivity. Therefore, attempt to form a conductive layer by electroless plating in stead of sputtering, thereby further reducing the costs and increasing the productivity, has been made.
Meanwhile a process of soaking a polyimide resin in an aqueous solution of an alkaline metal hydroxide has been known as a method for achieving electroless plating on the polyimide resin with excellent adhesion. The electroless plating adhesion is improved by using the process of soaking the polyimide resin in the aqueous alkaline metal hydroxide solution because, when the polyimide comes into contact with the solution, imide rings of the polyimide are alkali-hydrolyzed and opened to form polar groups, and as a result the chemical bonding to a metal film is increased. Further, it is presumed that the polyimide surface is etched and made rough by the aqueous alkaline metal hydroxide solution, whereby the contact area between the surface and the metal film is increased, and the adhesion is improved by the anchor effect.
Recently reported are a method described in Patent Document 1 containing the steps of treating a polyimide resin with an aqueous potassium hydroxide solution to open an imide ring, thereby generating a carboxyl group, coordinating a palladium or copper ion to the carboxyl group, applying a reducing agent to the resin, irradiating the resultant with ultraviolet ray through a photomask to reduce the palladium or copper ion in the irradiated region, thereby forming a catalyst metal core, and depositing an electroless plating layer to form a circuit pattern, and a method described in Patent Document 2 containing the steps of applying an alkaline alcohol aqueous solution by an ink-jet process to form a pattern, and adsorbing a catalyst metal only to a portion coated with the solution to deposit an electroless plating layer.
It is believed that, instead of conventional subtractive methods of etching the copper layer in regions unnecessary for forming circuits in a copper-clad polyimide substrate to form insulating portions, the fully additive methods of depositing a metal only in regions necessary for forming circuits will be used more widely with increase of wiring density and improvement of performance of printed circuit boards.
However, the above methods are disadvantageous in that palladium chloride, etc., which are widely used as catalyst metals for electroless plating, have poor adsorption selectivity and are adsorbed also to portions other than carboxyl groups. Thus, in view of responding the high wiring density expected to further increase in future, a technology for adsorbing a catalyst metal selectively to an anionic group such as a carboxyl group, thereby achieving electroless plating, is demanded.    [Patent Document 1] JP-A-2001-73159    [Patent Document 2] JP-A-2005-29735